This invention relates to means for detecting occurrence of signals and more particularly relates to threshold means for providing a time measure of the leading and trailing edges of a radiated signal incident at a receiver.
In various fields, such as radio navigation, threshold circuits are required to exclude extraneous signals from consideration and also to provide an indication of when a signal can be considered to start and stop. For example, in a microwave landing system for aircraft it is proposed to disseminate angular information, such as azimuth or elevation information, correlated to the location of a receiving aircraft with respect to a station at the air field by radiating a scanning beam from the station through a section of space from a known starting position to a known end position, and return at a known or constant rate. In this case, the time between subsequent passages of the beam at a receiving aircraft will be a measure of the azimuth or elevation, as appropriate, of the aircraft. This scheme is herein termed the time reference scheme. As an alternative, it is proposed to scan a beamed signal of predetermined frequency into space from a starting position to an end position. Encoded on the beam by frequency modulation is an angle sub-carrier whose frequency varies with beam pointing angle. In this case the mean frequency of the sub-carrier while the beam is incident on a point in space is a measure of the angular position of the receiver. This alternative scheme is herein termed the frequency reference scheme. In either case, it is necessary to accurately threshold the beam signal. Threshold circuits are used to determine when a signal occurs. Where only moderate accuracy is required, prefixed thresholds are satisfactory. Where greater accuracy is desired, storage circuits are provided which permit thresholding of a second occurring signal with a voltage derived and stored from the amplitude of a previously received signal. However, rapid signal fading, particularly that experienced close to the threshold, may cause amplitude changes in the detected signal envelope over the time period between consecutive signals and of a magnitude sufficient to cause loss of data. Such fades are normally caused by a combination of vertical and lateral signal multipath and are particularly noticeable when the receiver is moving rapidly.